1. Field of Invention
This invention relates to the use of microwave reflection resonator sensors for measuring or monitoring the complex dielectric or conductive properties of materials in situ.
2. Discussion of Prior Art
Microstrip resonator sensors are effective devices for measuring the complex dielectric constant .epsilon.* (=.epsilon.'-j.epsilon.") of materials at microwave frequencies as disclosed by Flemming U.S. Pat. No. 4,865,370), by Heath U.S. Pat. No. 3,510,764) and by Gerhard U.S. Pat. No. 3,942,107). Flemming describes a method in which a microstrip resonator is mounted on a copper-backed dielectric substrate. The resonator is weakly capacitively coupled to a microwave feed source and to a microwave detector so that the resonator Q-factor is not affected by the impedances of the source or the detector. When the test dielectric is placed near the resonator, the electromagnetic fields near the resonator are coupled to the test dielectric so as to affect the resonator's resonant frequency and Q-factor as measured by a detector for transmission between the separate sensor input and output ports. Further, special methods for modulating either the source frequency or the resonant frequency of the resonator are disclosed which avoids the need to sweep the source frequency through the resonance of the resonator.
Heath's invention uses a half wavelength microstrip resonator which is tightly sandwiched between two sheets of the sample test material. In turn, these sheets of test material are clamped in a special fixture. The microstrip resonator is loosely capacitively coupled to a microstrip line which passes near one end of the resonator normal to the resonator length. The dielectric constant is determined from measurements of the resonant frequency and Q-factor for transmission between the sensor's two input and output ports. Since special cutting and positioning of thin sheets of the sample material in the test fixture is required, Heath's method is not in situ or nondestructive.
Both Flemming and Heath's inventions involve transmission from an input port to an output port, both ports being loosely or weakly coupled to the intervening resonator by capacitive coupling. In comparison, the present invention utilizes the reflected wave from one port only, said single port being much more strongly coupled to the resonator. This coupling is, in fact, near critical such that the source port is reasonably well matched to the resonator at the resonant frequency. Besides being simpler through the elimination of one port, the near critical coupling feature of this present invention permits determining the real (.epsilon.') and the imaginary (.epsilon.") components of the test material dielectric constant simultaneously and independently. Moreover, near critical coupling permits measurement of the resonant frequency with exceptionally high accuracy and resolution. As a result, the present invention can resolve extremely small changes in .epsilon.', even when the Q-factor is low, i.e., when the test material is very lossy. Such measurements for low Q-factors are much more difficult or impossible when using such transmission resonators.
Gerhard's invention is a microstrip one-port reflection resonator sensor intended only for measurement of the real dielectric constant (.epsilon.') of thin microwave substrate material samples. The test materials are cut into samples suitable for fitting into a special fluid-(e.g., air) pressurized fixture for forcing the test dielectric against a thin substrate film on which the metal resonator is formed. Thus, Gerhard's invention is not in-situ or nondestructive. Moreover, there are many other distinct differences in the construction, operation, and the single use of Gerhard's invention compared to the broad range of uses of the present invention. In particular, Gerhard does not recognize the significance and importance of sensor losses in order to achieve near-critical coupling between the microwave source and the input to his invention. As noted above, this permits measurement of the dielectric loss factor (.epsilon.") and the real dielectric constant (.epsilon.') of the thin sheet of the test material with the greatest possible accuracy and resolution.
Besides measurement and monitoring the dielectric properties of materials using previously undiscovered microstrip reflection resonator sensor configurations, the present invention can also be used to measure or monitor the conductance of highly conducting materials. This application of microstrip resonators, operating in either the reflection or transmission modes, has not been previously discovered.